I should add that the good effects of frequent
intercrossing, and the ill effects of close interbreeding, probably
come into play in some of these cases; but on this intricate subject I
will not here enlarge.

Many cases are on record showing how complex and unexpected are the
checks and relations between organic beings, which have to struggle
together in the same country. I will give only a single instance,
which, though a simple one, has interested me. In Staffordshire, on
the estate of a relation where I had ample means of investigation,
there was a large and extremely barren heath, which had never been
touched by the hand of man; but several hundred acres of exactly the
same nature had been enclosed twenty-five years previously and planted
with Scotch fir. The change in the native vegetation of the planted
part of the heath was most remarkable, more than is generally seen in
passing from one quite different soil to another: not only the
proportional numbers of the heath-plants were wholly changed, but
twelve species of plants (not counting grasses and carices) flourished
in the plantations, which could not be found on the heath. The effect
on the insects must have been still greater, for six insectivorous
birds were very common in the plantations, which were not to be seen
on the heath; and the heath was frequented by two or three distinct
insectivorous birds. Here we see how potent has been the effect of the
introduction of a single tree, nothing whatever else having been done,
with the exception that the land had been enclosed, so that cattle
could not enter. But how important an element enclosure is, I plainly
saw near Farnham, in Surrey. Here there are extensive heaths, with a
few clumps of old Scotch firs on the distant hill-tops: within the
last ten years large spaces have been enclosed, and self-sown firs are
now springing up in multitudes, so close together that all cannot
live.

When I ascertained that these young trees had not been sown or
planted, I was so much surprised at their numbers that I went to
several points of view, whence I could examine hundreds of acres of
the unenclosed heath, and literally I could not see a single Scotch
fir, except the old planted clumps. But on looking closely between the
stems of the heath, I found a multitude of seedlings and little trees,
which had been perpetually browsed down by the cattle. In one square
yard, at a point some hundred yards distant from one of the old
clumps, I counted thirty-two little trees; and one of them, judging
from the rings of growth, had during twenty-six years tried to raise
its head above the stems of the heath, and had failed. No wonder that,
as soon as the land was enclosed, it became thickly clothed with
vigorously growing young firs. Yet the heath was so extremely barren
and so extensive that no one would ever have imagined that cattle
would have so closely and effectually searched it for food.

Here we see that cattle absolutely determine the existence of the
Scotch fir; but in several parts of the world insects determine the
existence of cattle. Perhaps Paraguay offers the most curious instance
of this; for here neither cattle nor horses nor dogs have ever run
wild, though they swarm southward and northward in a feral state; and
Azara and Rengger have shown that this is caused by the greater number
in Paraguay of a certain fly, which lays its eggs in the navels of
these animals when first born. The increase of these flies, numerous
as they are, must be habitually checked by some means, probably by
birds. Hence, if certain insectivorous birds (whose numbers are
probably regulated by hawks or beasts of prey) were to increase in
Paraguay, the flies would decrease--then cattle and horses would
become feral, and this would certainly greatly alter (as indeed I have
observed in parts of South America) the vegetation: this again would
largely affect the insects; and this, as we just have seen in
Staffordshire, the insectivorous birds, and so onwards in
ever-increasing circles of complexity.